1. Field of Invention
This invention pertains to electronic circuits, and more particularly to a voltage regulator fabricated utilizing only MOS transistors.
2. Description of Prior Art
In electronic circuits, it is often desired to provide a voltage source which is well regulated in order to provide a stable, relatively unvarying voltage source. In certain instances, this is desired because the power supply voltage may vary, for example under load or due to temperature changes, while a particular circuit is desired to be powered by a substantially more stable voltage. For example, in low voltage integrated circuit systems utilizing a supply voltage VDD of 3.3 volts, power supply variation of +/-5% is often the case.
Another problem compounding the difficulty associated with providing a highly stable voltage is the problem of noise. Such noise might be the result of poor filtering in the power source itself, or due to noise imposed on the power supply as it is distributed through a complex electronic system, such as an integrated circuit or a computer system utilizing a plurality of integrated circuits. Such noise problems are often exacerbated by the fact that digital signals operating at a high rate of speed, and therefore switching very rapidly, cause AC noise on the power supply due to transient voltages and rapid changes in power supply load currents. This AC noise is particularly problematic when analog circuitry, which is often highly sensitive to power supply voltage variations, is powered from the same power supply as is digital circuits. For example, in a typical computer system, a significant amount of digital circuitry is utilized, causing AC noise to be superimposed on power supply voltages. Such computer systems might also contain analog circuitry such as phase lock loops which are used for clock deskewing and synchronization with an external clock.
It is known in the past to filter and regulate supply voltages in order to reduce the effects of noise coming from either the power supply itself, or noise signals being superimposed on the distributed power supply voltage from other circuitry, such as rapidly switching digital circuits. Such prior art voltage regulators were formed utilizing bipolar devices, due to several advantages inherent in bipolar fabrication processes. First, in bipolar technology, PN junctions, with a reliable voltage characteristic, are readily available and can be formed so as not to necessarily be tied to either VDD or ground. In contrast, given the typical MOS or CMOS fabrication process, the junctions formed in MOS devices are tied either to VDD or to ground. Furthermore, in bipolar technology, diffused resistors of reasonable tolerances can be formed, unlike MOS processes in which diffused resistors are not normally formed and, if they are, have poor tolerances.
In the prior art, it is known to fabricate devices utilizing a BICMOS process, resulting in a single integrated circuit including bipolar devices and MOS devices. However, this BICMOS process is inherently more complex than either a bipolar or an MOS process, and thus one would not wish to use a BICMOS process unless there were significant needs for both bipolar and MOS circuitry on the same integrated circuit.
It is also known in the prior art to avoid the complexities of BICMOS processing by utilizing an MOS integrated circuit requiring a highly regulated supply voltage, with an external voltage regulator of some sort, such as a bipolar voltage regulator. However, this approach reduces the level of integration, thereby increasing circuit complexity, manufacturing complexity, and ultimate costs.